The term micro electromechanical system (MEMS) or micro mechanical system/structure is often used to refer to small integrated devices or systems that combine electrical and mechanical components. When focusing on the micro mechanical parts, the term “micro mechanical system” may be used to describe small integrated devices or systems which include one or more micro mechanical elements and possibly, but not necessarily, electrical components and/or electronic components.
Micro mechanical systems may be used as, for example, actuators, transducers or sensors. Micro mechanical systems or structures (MMS) may include deflectable structures such as membranes. A micro electromechanical structure (MEMS) may include one or more micromechanical structures whose deflectable structure may be deflected electrically (actuator). Alternatively or in addition, the MEMS may provide an electrical signal responsive to a deflection of the deflectable structure of the MMS (sensor).
Movable microstructures such as, for example, membranes for micro transducers (e.g., microphones or micro loudspeakers) or cantilevers for atomic force microscopes (AFM) may require, particularly at their suspension points or areas or at areas that are important for the functionality of the structures, materials that allow to modify both the static bending characteristics/deflection behavior and the dynamic behavior under resonance conditions depending on the specific application.
Very soft AFM cantilevers (for example, made of a polymer) may result in a low resonant frequency, which in turn reduces the maximum scanning speed and sampling rate. For example, in the field of mechanically sensitive samples, e.g., for a measurement of organic substances like organic cells, very soft AFM cantilevers may reduce the risk to damage the samples. Furthermore, a laser may not be used in a liquid for readout of the oscillation/bending, but a sensor element (mostly a piezo resistive strain sensor) that is integrated on the cantilever beam may be required. In case of very soft materials, though, this may have the effect that due to the extreme softness of the base material, extension and compression of the base material that may be transferred onto the cantilever beam, is only detected to a very limited amount due to the high mechanical stiffness of the sensing material (e.g. in case of a strain sensor).
Soft polymers such as, for example, SU8 may furthermore have a limited stability under environmental conditions. For example, they may exhibit high shrinkage during manufacturing, water intake during operation, and/or aging phenomena. Furthermore, they may have a very high coefficient of thermal expansion (CTE) compared to standard semiconductor materials (e.g., SU8 has a CTE of about 52 ppm/° C. compared to Si having a CTE of about 2.56 ppm/° C.), and there may be only few possibilities to influence their elastic properties (e.g., the elastic modulus of SU8 is about 2 GPa).